Summary
A one-stage factor-VIII assay is described, based on the method of Lang-dell, Wagner
and Brinkhous (1953), in which contact activation is controlled by the addition of
a suspension of kaolin to the system. Experiments to show the effect of this modification
on the clotting times and reproducibility of the system are presented.
The error of the method is expressed in terms of the confidence limits of a random
series of field assays, which lay between 64—156% and 76—131% of the observed result.
The normal range of factor-VIII concentrations, as determined by this method on venous
plasma samples from 116 normal subjects, was 50—170%, with a roughly normal distribution.
There was good correlation between the results of one-stage and two-stage factor-VIII
assays carried out in parallel on 110 plasma samples and between the observed and
expected factor-VIII concentrations of a series of mixtures of normal and haemophilic
plasma.
The use of the assay on capillary plasma samples is described, and the results compared
with those obtained on venous plasma. The practical value of capillary factor-VIII
assays is discussed.
